Logically hons-copy-persistent is the identity; we leave it enabled and
would think it odd to ever prove a theorem about it.

Under the hood, hons-copy-persistent is virtually identical to
hons-copy.

The only difference is that when hons-clear is used, any persistently
normed conses are automatically re-normed, and this re-norming can be carried
out more efficiently than, say, an ordinary hons-copy.